Field of the Invention
The invention relates to an apparatus and method of combustion that produces low emissions, carbon monoxide and oxides of nitrogen in particular, and to progressing reactions to produce low intermediate products and byproducts.
Description of the Related Art
Energy conversion and chemical processing industries, seek to economically remove and/or produce specific chemical species. Unburned hydrocarbons (UHCs), carbon monoxide (CO), and oxides of nitrogen (Nox) are three sets of chemical species which are commonly found in energetic fluids formed in hot chemical reactions, and more particularly, in combustion-based energy conversion industries.
Legislative authorities have periodically reduced allowable emission levels of these pollutants. Manufacturers of combustion-based power and other energy-conversion systems thus seek improved pollutant reduction systems. Methods of reducing the emissions of these and other common pollutants typically include,
(1) modifying the combustion process itself, e.g., tuning or configuring the combustor/burner section of the system, or adding reactants to reduce the emissions (such as ammonia), and
(2) utilizing add-on technologies that effectively reduce or remove pollutant species produced in earlier phases of the overall chemical or energy-conversion process.
In typical combustion systems, control measures aimed at controlling each of NOx, CO and UHCs can be counterproductive, thus increasing the cost and complexity of controlling overall emissions. For example, NOx emissions are understood to increase with, (1) increasing combustion temperatures, becoming especially important above about 1300°-1500° C., (2) with increasing residence time at the NOx-producing temperatures, and (3) with increasing concentrations of the effective oxidant, typically O2, at the NOx-producing temperatures. Consequently, some of the most common in-situ or “in-combustor” strategies for reducing NOx levels are informed by these understanding. They sometimes involve adding a diluent, such as excess air, exhaust gas, steam or water, to reduce undesirably high temperatures in the combustor. Furthermore, the more upstream the diluent is delivered to the combustor, the shorter the residence time at NOx-producing temperatures. In some technologies diluent air and/or steam is premixed with the fuel and/or oxidant containing fluid to constrain the peak reaction or combustion flame temperatures. To reduce the amount of oxidant available that would oxidize nitrogen-containing compounds to form NOx, diluents other than air, oxygen or similar oxidants are used.
It is commonly expected that CO and UHC emissions increase with decreasing overall combustor temperatures and reduced residence times at temperatures high enough to promote UHC and CO oxidation to CO2. Consequently, diluent-based NOx reduction strategies typically result in simultaneous undesired increases in the CO and UHC emissions in the energetic or working fluid leaving the combustor, and vice versa. Furthermore while higher temperatures seem to promote UHC removal, CO emissions tend to increase with incremental increases in temperature in high temperature ranges. This is especially a problem as higher turbine inlet temperatures are sought for higher efficiency in gas turbine systems. The CO levels produced from systems with high combustor exit/Turbine Inlet temperatures are often substantially higher than legislatively allowed or desired emission levels.
Efforts to increase system throughput typically result in shorter hot residence times, further raising UHC and CO concentrations. Rapid expansion through a work engine or expander such as a turbine typically results in rapid reductions in fluid temperature that “freeze” or “quench” the conversion of UHC and CO to CO2, which can result in high UHC and CO emissions.
As in-situ methods typically have limited success in reducing emissions, they are often combined with add-on technologies that reduce or remove pollutant species produced and present in the combustion product gases, e.g. selective catalytic reduction (SCR). Such add-on techniques typically process the exhaust gases at low pressures. The utilized add-on technologies often substantially increase the volume, footprint and cost of the systems because of the high specific volume and comparatively long kinetic time-scales of reactions that characterize exhaust gases. Thus, even though these methods may successfully reduce emissions to acceptable levels, they often substantially increase equipment, operation and maintenance costs.